For many vegetable gardeners, the season is over with the first heavy frost. But many of our better-known vegetables are hardy, and easy to grow and harvest right up to the early winter months. Proper sowing-time is the key.

Brussels sprouts, kale and collards can stand short periods of cold, as low as 10 degrees, and their flavor will actually improve. Collards and kale often make it through winter and add new growth in spring: Sow seeds before August 10. Seedlings of Brussels sprouts should be planted before August 1. Garden centers should have them.

Beets and carrots will usually last well into November without a covering. Seeds should be sown before August 1. After harvesting, these root vegetables can be buried in a well-drained location where they may last until the spring thaw.

Winter radishes can be handled the same way as beets and carrots, but should not be sown until late August.

Seedlings of cabbage (red and Savoy types are best for winter use) can be planted up to August 20. Don't handle heads while they are frozen: Wait for a thaw, then harvest.

Broccoli, cauliflower, kohlrabi and Chinese cabbage usually will yield into late November. Seedlings of broccoli and cauliflower should be planted in late tjuly, and seeds of kohlrabi and Chinese cabbage by August. Hard freeze ruins compact Chinese cabbage, but loose-leaf types survive several freezes.

Turnips and rutabagas provide good eating all fall and well into winter. Cover the rows in the late fall with a thick layer of straw or evergreen boughs, and when weather permits, push aside the covering and harvest as many roots as you want. Sow rutabaga seeds before August 1 and turnip seeds before September 1.

Winter Bloomsdale spinach survives even in severly cold areas without any protection: Sow before September 1.

Leaf lettuce and head lettuce stand some frost in the garden and, if grown protected in a cold frame, can be harvested well into winter. Sow seeds of leaf lettuce before September 1 and seedlings of head lettuce before August 15. It sometimes helps to cool lettuce seed in the refrigerator for two or three days before planting, to help it sprout.

Once they sprout, the young plants will probably require watering to keep them doing well. And before frost, worms and insects may be a problem to all the greens. Careful inspection and washing is always a good idea.

Dry soil may inhibit good germination of seeds during midsummer. Sow them when the soil is moist and cover them twice as deep as you did in spring. CAPTION: Picture, NOW'S THE TIME TO START TO SEE THESE THIS FALL.